1. Field of the Invention
The invention relates to a semiconductor device having a function to control with a transistor a current supplied to a load. In particular, the invention relates to a pixel formed of a current drive type light emitting element of which luminance changes by a current, to a display device including a scan line driver circuit and a signal line driver circuit, and to a driving method thereof. Further, the invention relates to an electronic device having the display device in the display portion.
2. Description of the Related Art
In recent years, what is called a self-luminous type display device, which has pixels formed of light emitting elements such as light emitting diodes (LEDs), is attracting attentions. As a light emitting element used for such a self-luminous type display device, an organic light emitting diode (OLED), an organic EL element, and electroluminescence (also referred to as an electroluminescence (EL) element) are notable and becoming to be used for an EL display and the like. A light emitting element such as an OLED which is a self-luminous element is advantageous in that visibility of pixels is high, a backlight is not required, response is fast, and the like as compared to a liquid crystal display. Luminance of a light emitting element is controlled by a current value flowing therethrough.
As a driving method to express a gray scale of such a display device, there are an analog gray scale method and a digital gray scale method. The analog method includes a method to control light emission intensity of a light emitting element in an analog manner and a method to control light emission time of a light emitting element in an analog manner. The analog gray scale method often employs a method to control light emission intensity of a light emitting element in an analog manner. However, the method to control light emission intensity in an analog manner is easily affected by variations in characteristics of a thin film transistor (hereinafter also referred to as a TFT) of each pixel, which leads to variations in light emission of each pixel. In the digital gray scale method, on the other hand, a light emitting element is controlled to be turned on/off in a digital manner to express a gray scale. In the case of the digital gray scale method, the uniformity in luminance of each pixel is excellent; however, only two gray scale levels can be expressed since there are only two states: light emission or non-light emission. Therefore, another method is used in combination to realize a multi-level gray scale. There is an area gray scale method to express a gray scale by selecting the weighted light emission areas of pixels and a time gray scale method to express a gray scale by selecting the weighted light emission time. In the digital gray scale method, a time gray scale method which is suitable for achieving high definition is often employed.
[Patent Document 1]
    Japanese Patent Document Laid-open No. 2001-343933